Directional couplers are generally used to split and combine radio frequency (RF) signals and other signals among multiple nodes corresponding to signal sources and/or endpoints. Directional couplers are commonly used for “in building” signal distribution between a central source point and multiple endpoints. In these and other applications, directional couplers are used to split a fixed amount of power from a “trunk” input between different “branch” outputs.
A conventional directional coupler 10 is illustrated in FIG. 1. The conventional directional coupler 10 includes a main coaxial line 12 extending between input port 14 and transmitted port 16, and a coupled coaxial line 18 extending between coupled port 20 and isolated port 22. In conventional directional couplers, such as directional coupler 10 in FIG. 1, the main line 12 and the coupled line 18 are arranged in close proximity. This arrangement permits a portion of input power applied to the input port 14 to be diverted, or “coupled,” to the coupled port 20, based on the inductance of the main line 12 and the coupled line 18, and is output at ninety degree (90°) phase to the input signal. The remainder of the input power is transmitted to the transmitted port 16 in phase with the input signal (i.e., at zero degree (0°) phase).
In many applications, the isolated port 22 is connected to a matched load 24 (e.g., a 50 ohm resistor), which is connected to ground to dissipate any power that is reflected back toward the isolated port 22. This matched load 24 can be internal to the directional coupler 10, with the isolated port 22 not accessible to a user, effectively resulting in a three port device.
Conventional directional couplers 10 have a fixed “coupling factor,” measured in decibels (dB), that is based on the ratio of the power output at coupled port 20 (Pcoupled) to the power input at input port 14 (Pinput) within a given frequency range. Coupling factor C can be expressed as Equation 1 below.
                    C        =                              -            10                    ⁢                                          ⁢                      log            ⁡                          (                                                P                  coupled                                                  P                  input                                            )                                ⁢          dB                                    Equation        ⁢                                  ⁢        1            
Thus, for a directional coupler 10 that diverts fifty percent (50%) of its input power from input port 14 to the coupled port 20, the coupling factor is −3 dB. −3 dB directional couplers, also known as hybrid couplers, are a common design because they split an input signal from input port 14 equally, with half of the input power being coupled to coupled port 20 and the other half of the input power being transmitted to transmitted port 16. A −6 dB directional coupler is another common type of coupler that couples twenty five percent (25%) of the input power to the coupled port 20 and transmits seventy five percent (75%) of the input power to the transmitted port 16. Likewise, a −10 dB directional coupler couples ten percent (10%) of the input power to the coupled port 20, and transmits ninety percent (90%) of the input power to the transmitted port 16.
Because hybrid couplers are bi-directional, they can be used to coherently combine power in addition to splitting it. For example, when a signal is applied to coupled port 20 of a directional coupler 10 having a coupling factor of −3 dB (a hybrid coupler), and the same signal is applied to the transmitted port 16 at ninety degree (90°) phase to the signal applied to coupled port 20. The signals add to each other at the isolated port 22 because the directional coupler 10 causes both signals output at the isolated port 22 to be in phase with each other. Likewise, the signals output at the input port 14 cancel each other out because the directional coupler 10 causes the signals to be at one hundred eighty degree (180°) phase to each other. Likewise, if the signal applied to transmitted port 16 is instead at ninety degree (90°) phase to the signal applied to the coupled port 20, the signals add at the input port 14 and cancel each other out at the isolated port 22.
Coupling factor C is centered around a particularly rated frequency band. Although the coupling factor C can be made to be relatively flat within the rated frequency band, coupling nevertheless varies with frequency. Therefore, couplers are specified in terms of the coupling accuracy at the frequency band center. However, for common applications, such as RF signal distribution, the coupling factor C of directional couplers 10 can be considered to be effectively constant for those applications.
Coupling factor C is a negative quantity that cannot exceed 0 dB for a passive device such as conventional directional coupler 10. In practice, a coupling factor C does not typically exceed −3 dB since a coupling factor C between −3 dB and 0 dB would result in more power output from the coupled port 20 than power from the transmitted port 16, effectively, reversing the roles of the coupled port 20 and transmitted port 16.
Additional discussion of directional couplers can be found at, for example, en.wikipedia.org/wiki/Power_dividers_and_directional_couplers (accessed Feb. 1, 2013), U.S. Pat. No. 8,258,889, filed on Nov. 30, 2009 and U.S. Pat. No. 5,689,217, filed on Mar. 14, 1996, which are hereby incorporated by reference herein in their entirety for this purpose.